It has become apparent that battery systems will be increasingly used in the future, both in stationary applications and in vehicles, such as hybrid and electric vehicles. In order to meet the requirements of voltage and potentially available power specified for a respective application, a high number of battery cells are connected in series. Due to the series connection, the failure of one battery cell may lead to the failure of the battery and in turn to a failure of the entire system, wherefore high demands are placed on the reliability of the battery, in particular for safety-relevant applications. In order to detect the state of the battery and of the individual battery cells as accurately as possible and to thus identify a risk of failure of a battery cell in good time, the voltages of the battery cells in particular are measured regularly in addition to other parameters of the battery or battery cells. In the prior art, numerous methods are known to carry out what is known as cell balancing, that is to say to attempt to adapt the charge states of the various battery cells of a battery to one another, which has a positive effect on the service life of the battery. To this end, the battery is equipped with units that determine the cell voltages of the individual battery cells and also optionally further measurands, such as battery temperature and battery current, and transmit these to a central control unit (for example a microcontroller). The control unit calculates for each battery cell, from the transmitted measurement data, a respective charge state and generates the control signals necessary for carrying out the cell balancing process, said control signals causing the charge balancing among the battery cells in the battery, for example by partially discharging battery cells having a higher charge state. Here, the control unit is arranged outside the battery in order to take on further tasks. This entails the disadvantage however that relatively complex safety measures, such as insulators arranged in the data path, have to be taken in order to prevent, in the event of a fault, a short circuit between the high battery voltage (often several hundreds of volts in the case of battery-operated drive systems) and the parts, such as the control unit, which are arranged in the low-voltage network and can be contacted from the outside.
The present disclosure proposes a charge balancing unit which can adapt to one another the charge states of two battery cells connected in series and is illustrated in FIG. 1. To this end, a voltage divider is connected parallel to the two battery cells and halves the total voltage of the two battery cells connected in series and thus provides a reference voltage. This reference voltage is compared with the voltage at the connection point between the two battery cells connected in series, for which purpose a simple differential amplifier or comparator can be used. The output of the differential amplifier is connected to the connection point. The supply voltage connectors of the differential amplifier are connected to the highest and the lowest potential of the two battery cells.
If there is now a differential voltage at the inputs of the differential amplifier because the cells voltages of the two adjacent battery cells deviate from one another, such that the voltage of the connection point is not exactly equal to the reference voltage, the differential amplifier will output an output current having an appropriate sign, which counteracts the cause of the differential voltage, that is to say discharges the battery cell that has a higher cell voltage.
In addition, a possibility has been proposed to connect pairs of such charge balancing units, whereby any numbers 2^n of battery cells can be connected by recursive connection (FIG. 2). The limitation to powers of two is less desirable, however. In addition, the voltage dividers connected into the charge balancing units, of which increasingly more are provided in parallel with an increase in the number of battery cells, signify an undesirably high loading of the battery.